As personal computers (hereafter referred to as PCs) become increasingly popular, there is growing demand for the display of PC signals on larger screens using inexpensive means. In general, multi-scan displays and other exclusive displays are used as video display monitors for displaying PC signals. However, large multi-scan displays and other exclusive displays cost a great deal more because of the advanced technology used in these displays.
On the other hand, high definition television sets (hereafter referred to as HDTVs), which are capable of displaying PC signals, are rapidly penetrating the market. Consequently, demands for displaying PC signals on HDTVs are sharply increasing. However, HDTVs are unable to display PC signals directly. An interface is required to link a PC with an HDTV for displaying PC signals on HDTVs. For example, a video signal converter as disclosed in Japanese Laid-open Patent H8-9343 is required. With the use of this video signal converter, PC signals can be displayed on HDTVs. The video signal converter of the prior art is explained with reference to FIG. 5. FIG. 5 is a block diagram of the video signal converter of the prior art.
In FIG. 5, an input signal processor 51 converts the analog input signal to a digital signal, and separates and outputs the horizontal synchronizing signal (HD1) and vertical synchronizing signal (VD1) of the input signal. The input signal processor 51 also produces a clock pulse (CK1) which is used for converting the input signal to a digital signal.
A compressor 52 condenses the number of picture elements in horizontal and vertical directions in a specified ratio as required, and passes through the signal unmodified when compression is not necessary. A memory 53 provisionally stores data. An expander 54 enlarges the number of picture elements in the horizontal and vertical directions in a specified ratio as required, and passes through the signal unmodified when expansion is not necessary.
An output signal processor 55 conducts necessary signal processing including conversion of the digital signal to an analog signal. A controller 56 outputs a compression or expansion ratio for the horizontal or vertical direction for converting and outputting the input signal in a required form of signal. The required form of the signal is the display format for displaying the signal. The controller 56 also outputs a signal for controlling write and readout of the memory 53. An output HD2/VD2/CK2 generator 57 produces and outputs the output horizontal synchronizing signal (HD2) and output vertical synchronizing signal (VD2) after conversion. The output HD2/VD2/CK2 generator 57 also outputs a clock pulse CK2.
The operation of the video signal converter of the prior art configured as illustrated in FIG. 5 is described below. In order to convert the input signal to a different format, the video signal converter compresses or expands the input signal and then outputs a synchronizing signal in a required form. The video signal converter in FIG. 5 compresses the input signal using the compressor 52 and expands a signal from the memory 53 using the expander 54. Both compression and expansion is performed during digital signal processing.
For example, assume the input signals are of the NTSC system, extended definition television standard (hereafter referred to as EDTV standard), high definition television standard (hereafter referred to as HDTV standard), or PC signals of video graphics array (hereafter referred to as VGA specification). Further, it is assumed that the input signals after conversion are also of the NTSC system, EDTV standard, HDTV standard, or PC signals of VGA specification. Compression and expansion of these signals are determined based on the correlation of the input and output signals. When compression is necessary, the video signal converter of the prior art compresses the signal using the compressor 52 and provisionally stores it in the memory 53. Data are then read out from the memory 53 synchronized with CK2 and are passed through the expander 53 without undergoing expansion processing. When expansion is necessary, the video signal converter of the prior art passes the signal through the compressor 52 without undergoing compression processing, and provisionally stores it in the memory 53. Data are then readout from the memory 53 synchronized with CK2 and are expanded by the expander 53. If the input signal is the PC signal of VGA specification, for example, and the form of signal required after conversion is the HDTV signal, the vertical frequency of the input signal is 60 Hz and that of the output signal is 59.94 Hz. The difference in the vertical frequency between the input and output signals is converted by controlling write and readout of the memory 53.
As described above, the video signal converter of the prior art conducts all compression and expansion processes during digital signal processing. Therefore, in order to compress and expand the input signal without any deterioration, it is necessary to provide a sufficient number of taps in compression and expansion filters so that signals are regenerated without distortion due to conversion of the sampling frequency. To satisfy this requirement, a very large-scale circuit is necessary. Moreover, if the input signal is the PC signal of VGA specification and the signal required after conversion is the HDTV signal, the vertical frequency of the input signal is 60 Hz and the vertical frequency of the output signal is 59.94 Hz. To convert between these two frequencies the video signal converter of the prior art uses a large-scale digital circuit for interpolation or the control of write and readout of the memory 53. Consequently, deterioration in picture quality due to interpolation or missing images may occur.